1. Field of the Invention
The present invention relates to thermal and thermal transfer label printers and, more specifically, to such printers adapted to printing on pressure sensitive adhesive-backed labels.
2. Description of the Prior Art
Thermal and thermal transfer printers are well known in the art. In thermal label printers, a web of pressure sensitive adhesive-backed labels, each having a thermally sensitized surface, is fed between a platen roller and a thermal print head. In a thermal transfer label printer, a transfer ribbon having a heat transferrable ink layer is additionally interposed between the print head and the label so that non-sensitized labels may be printed. The transfer ribbon is flexible and typically no thicker than ten microns Thus, the principles of the present invention are equally applicable to thermal and thermal transfer printers.
Pressure sensitive adhesive-backed labels for automated printing are typically presented in a continuous web. The web consists of a backing sheet of wax or silicone-impregnated paper approximately 0.0015″ thick and having multiple labels of paper, polyester, synthetic paper, or similar material having a thickness between 0.0015″ and 0.010″ removably mounted thereon with a rubber or acrylic pressure-sensitive adhesive. Successive labels are separated by an interlabel gap, typically 0.125″ wide, to which the printer is responsive for alignment of printing on the label. The web may be supplied from, for example, a roll or a fanfold.
In a friction fed thermal printer, deformation of the platen roller and slippage between the backing material and the platen introduce variability in the feed distance of the web per increment of platen shaft rotation. Slippage is a function of the web tension and produces a net loss in web advance, for example in an on-demand printer, when the printer advances a label against supply roll inertia to facilitate clearance from the printer for individual removal after printing and then backfeeds into a slack web before printing the next label.
Any error in web advance accumulates as successive labels are printed, resulting in progressive misregistration of the label image with respect to the label edges. A friction fed printer thus requires some means of sensing the edge of each label for synchronization in order to print multiple labels without manual intervention.
Label location in typical prior art thermal printers has been accomplished by measuring the optical transmissivity of the web. The backing is illuminated by a light source of known intensity, typically an infrared light-emitting diode. The amount of light passing through the backing between labels is greater than the light passing through the laminated backing and label. The transmitted light illuminates a photocell, which converts the changes in transmitted light to a varying electrical signal. The electrical signal can then be measured and interpreted as the label edge location by the printer's logic circuits and used to synchronize printing of each label.
However, optical sensors have inherent limitations. Even though the intensity of the light source is constant, the paper fibers in the label and in the backing produce fluctuations in the light intensity that may introduce errors into the edge determination. Because the light source and or optical sensors are proximate the media path, they may be fouled over time by fibers or other detritus continuously introduced into the media path by the media. Also, a transverse movement of the slack web perpendicular to its plane between the light source and the photocell may occur during backfeed, introducing an additional error.
The optical sensor is typically located an inch or more away from the heater elements to avoid mechanical interference with the print head or platen. If the web slips between the time the leading edge of a label passes the photocell and when it reaches the heater elements, or if slack develops between the photocell and the heater elements during backfeed, the printing will be misregistered on the label.
Applicant's U.S. Pat. No. 5,978,004 (Prior Art FIGS. 1–3) describes a label printer with a, for example, piezo-electric label edge sensor mounted on the print head or a bracket or support that carries the print head. This label edge sensor configuration advantageously allows direct sensing of the displacement caused by the arrival of each label edge at the print head. However, this configuration also has several limitations. First, signals from the sensor may have a poor signal to noise ratio due to support structure flexure and or external vibrations transmitted to the print head by the printer frame, print head pivot connection and or the print head spring. Second, if the print head spring strength and or spring mounting position(s) vary, the sensor response characteristics are changed. Further, use of different sized and or oriented labels, for example edge or center justified labels, may change the sensor response characteristics.
Competition in the market for label printers has focused attention on minimization of overall costs, including reduction of materials, manufacturing, operation and maintenance costs.
Therefore, it is an object of the invention to provide a system and method which overcomes deficiencies in the prior art.